Forms and profile distribution of soil phosphorus in four wetlands across gradients of sand desertification in Northeast China

Little is known about the relationship between soil phosphorus (P) forms of semiarid wetlands and surrounding upland land-use patterns. Soil P in different depths of four wetlands was fractionated using a modified Hedley fraction method. The wetland soil cores were sampled along a gradient of sand desertification in the west of Songnen Plain (Northeast China), and sectioned with high-resolution (2-cm intervals). The soil total P (Pt) content and P forms varied significantly within and among the profiles with strongly alkaline (pH 7.9 to 10.5) in sand and no sand desertification wetlands. The Pt mean content ranged from 90.8 to 231.6 mg/kg in the sand desertification wetlands, and 421.3 mg/kg in the no sand desertification wetland. Dilute HCl extractable inorganic P (Pi) is the predominant form in all profiles, both as absolute values and as a percentage of total extracted Pi (mean: 56–77%), and concentrated HCl extractable organic P (Po) is the predominant form of total extracted Po (mean: 45–68%). Moreover, Dil.HCl–Pi and Conc.HCl–Pi were strongly positive correlated with the content of Al, Ca, Fe, clay and silt percentages, but negative correlated with sand percentages. All forms of organic P (NaHCO3–Po, NaOH–Po, and Conc.HCl–Po) were strongly positive correlated with the content of total organic carbon (Corg). In addition, for comparison of soil heterogeneity affected by sand desertification, uplands surface soil samples were collected from grassland, cultivated land, and sand lands surrounding the four representative wetlands. The results indicated that P forms of the surface soils were significantly different among studied upland land-use patterns. Consequently, there were general differences in soil P forms between sand and no sand desertification wetland profiles. The Residual–P heterogeneity in the wetland profiles can partly be attributed to the influences of different intensity of sand desertification. And Residual–P could be as an index of the intensity of desertification.